Download Radioactivity Notes Day 1 and 2 Apr 23 and Apr 24

Science 10 – Notes Radioactivity
Radioactivity and Its History
• Radiation is everywhere, but can be difficult to detect.
o Discovery of cathode ray tube.
o Roentgen named X-rays with an “X” 100 years ago
because they were previously unknown.
o Becquerel placed uranium on photographic plate and
saw that it exposed the plate .
o Marie Curie and her husband Pierre named this energy
radioactivity.
Radioactivity and the Nucleus
 Rutherford Experiment (Gold foil experiment)
o He discovered that the nucleus of an atom is both
 Small
 Dense
 Isotopes are different atoms of the same element, with the
difference between the two atoms being the number of
neutrons in the nucleus.
o Eg. Carbon-12, Carbon-13, Carbon-14
o By having different numbers of neutrons, isotopes
have different mass numbers.
 Isotopes of an element have the same symbol
and same atomic number

 Atomic mass = proportional average of the mass
numbers for all isotopes of an element.
 19.9% of boron atoms have 5 neutrons,
80.1% have 6 neutrons
 19.9% have a mass number of 10, and
80.1% have a mass number of 11
 (.199 * 10) + (.801*11) = 10.8 = atomic mass
of boron
Representing Isotopes
 Isotopes are written using the following rule
o Chemical symbol + atomic number + mass number.
o Potassium has three isotopes,
39
40
41
19
19
19
 Potassium is found in nature in a certain ratio of isotopes
o 93.2% is potassium-39, 1.0% is potassium-40, and 6.7%
is potassium-41
o Atomic mass = (.932 x 39) + (.001 x 40) + (.067 x 41) =
39.1
K,
K,
K
Read pgs 275-282 Do 6, 10 pg 279 and do 3,9,11,13 pg 283
Day 2
Radioactive Decay: pgs 284-289
o Unlike all previously discovered chemical reactions,
radioactivity sometimes resulted in the formation of
completely new atoms.
o
o When these nuclei lose energy and break apart, decay
occurs.

 Radioactive atoms release energy until they
become stable, often as different atoms.

 These are called radioisotopes
Three Types of Radiation

o Positive alpha particles were attracted to the negative
plate.
o Negative beta particles were attracted the the positive
plate
o Neutral gamma particles did not move towards any
plate.
Alpha Radiation
o Alpha radiation is a stream of alpha particles, .
o Composed of 2 protons and 2 neutrons (no
electrons)
o Alpha particles are essentially the same as a helium
atom’s nucleus.
o
4
2
 or 42 He
o Because it has two protons but no electrons, it
has a charge of 2+.
o Alpha particles are slow and penetrate materials much
less than the other forms of radiation. A sheet of paper
will stop an alpha particle.
226
88
Ra 
222
88
Rn + 42 
or
226
88
Ra 
222
88
Rn + 42 He
Beta Radiation
o Beta radiation, , is an electron.
o Beta particles are represented by the symbols:
131
53
I 
131
54
Xe +
0
–1

or
131
53
I 
131
54
Xe +
0
–1
e
o Electrons are very tiny, so beta particles are
assigned a mass of 0.
o Beta decay occurs when a neutron changes into a proton
+ an electron.
o Beta radiation penetrates better than alpha
particles
o It takes a thin sheet of aluminum foil to stop a beta
particle.
Gamma Radiation
o Gamma radiation, , is a ray of high energy, shortwavelength radiation.
0
o Gamma radiation has no charge and
0
no mass,

o Gamma radiation is the highest energy form of
electromagnetic radiation.
 It takes thick blocks of lead or concrete to stop
gamma rays.
o Gamma decay results from energy being released
from a high-energy nucleus.
238
92
o
o
60
28
U 
Ni* 
Read pgs 284-288 Do 1-4, 6 pg 289
234
90
Th + 42 He + 2
60
28
Ni + 00
Radiation and Radioactive Decay Summaries, and Nuclear equations for radioactive decay
o
Nuclear equations are written like chemical equations, but represent changes in the
nucleus of atoms.
o
o
o
1. The sum of the mass numbers should equal.